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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | -1 |
| 2 | 1 |
| 3 | 0 |
| 4 | 3 |
| 5 | 4 |
| 6 | 20 |
| 7 | 50 |
| 8 | 189 |
| 9 | 588 |
| 10 | 2100 |
| 11 | 7116 |
| 12 | 25344 |
| 13 | 89298 |
| 14 | 321178 |
| 15 | 1156298 |
| 16 | 4206059 |
| 17 | 15356796 |
| 18 | 56424836 |
| 19 | 208137800 |
| 20 | 771229684 |
| 21 | 2867771004 |
| 22 | 10700980956 |
| 23 | 40050890172 |
| 24 | 150328400292 |
| 25 | 565699287186 |
| 26 | 2133889856550 |
| 27 | 8067040670100 |
| 28 | 30559571239890 |
| 29 | 115986196679730 |
[0,-1,1,0,3,4,20,50,189,588,2100,7116,25344,89298, 321178,1156298,4206059,15356796,56424836, 208137800,771229684,2867771004,10700980956, 40050890172,150328400292,565699287186, 2133889856550,8067040670100,30559571239890, 115986196679730] |